Thank you for the great question @YenK
There is nothing to worry about per se with regards to the sugar. The shakes don't stay in your mouth long (e.g. vs an apple) and the sugar (simple carbohydrates) are still maltodextrin and isomaltulose (still linked/complex) in your mouth.
What you've probably also seen online is warnings around liquid diets that consist of a lot of fruit (juices). One very important difference is that those are very acidic, Queal is neutral. 
That being said. One thing that we've been aware from since the beginning is that when someone lives of a 100% liquid diet there is a chance that you will not make enough saliva (of which production is activated whilst chewing), so your teeth will be less protected.
One suggestions is to add some chewing gum to your routine which simulates this behaviour (and is good for the jaw muscles).
In my experience here at the office (4 years +, 70% of calories from Queal), I find that my tooth health is still perfect.
